1. Field of the Invention
The present invention relates to colored, shaping thermoplastic resin pellets to be used for obtaining colored shaped articles of thermoplastic resins, and to a method for preparing them.
2. Description of the Related Art
In general, as shaping methods for obtaining colored shaped articles of thermoplastic resins, there are known 1 a method of feeding shaping thermoplastic resin pellets containing a powdery colorant into an injection-molding machine or an extrusion-molding machine, mixing them in the kneading step in the shaping machine and shaping the resulting mixture by injection molding or extrusion molding, 2 a method of gradually adding a liquid colorant or a master batch colorant to shaping thermoplastic resin pellets in an injection-molding machine or an extrusion-molding machine so as to mix and knead them and shaping the resulting mixture by injection molding or extrusion molding, and 3 a method of previously preparing colored, shaping thermoplastic resin pellets as an intermediate product and introducing the colored, shaping thermoplastic resin pellets into a shaping machine to shape them.
A conventional method for preparing colored, shaping thermoplastic resin pellets as the intermediate product to be used in the above-mentioned method for obtaining colored, shaped articles of thermoplastic resins is such that uncolored resin pellets, a powdery colorant and a blend oil such as a liquid paraffin are mixed in a mixer, then the resulting mixture is fed to an extruder so as to knead it in a hot molten condition therein, the resulting melt is line-wise or sheet-wise extruded into a water tank so as to cool it therein, and this is granulated into granular or cubic, colored pellets by cutting.
Such colored, shaping thermoplastic resin pellets have various advantages in that they may easily be fed to a shaping machine where they are shaped into colored, shaped articles as final products, they may be stored or kept in stock and they may be distributed as they are. Therefore, they are intermediate products which are extremely easily handlable.
However, colored, shaping thermoplastic resin pellets to be prepared by the above-mentioned conventional method have various problems such as those mentioned below. Precisely, according to the above-mentioned conventional method, in order that the powdery colorant and the shaping thermoplastic resin are integrated, they have to be in a melting state and then be granulated. Therefore, in order to attain the storability to prevent the powdery colorant from dropping off from the thermoplastic resin, equipments for melting and granulating the shaping thermoplastic resin and the running costs for them are indispensable, resulting in increased production costs. In addition, when the shaping thermoplastic resin is melted, it is heated at 200.degree. C. or higher so that its physical properties are deteriorated or it is often partly scorched to give scorched impurities to the resin melt. When the resin melt is extruded into a water tank, the resin is often hydrolyzed and deteriorated. When plural kinds of resin pellets having different colors are desired to be prepared, plural colorants with different colors are separately mixed with a thermoplastic resin one after another in the same mixer. Therefore, in order that the previous colorant remained in the mixer does not contaminate the next colorant with a different color, the inside of the mixer and the extruder to which the blend oil has adhered must be carefully cleaned at every change of the colorants, using solvents, detergents, waste or the like, but such cleaning operation is extremely troublesome.
In order to solve these problems inevitable in preparing colored, shaping thermoplastic resin pellets by the conventional methods, the present inventor expected that, if organic solvents and organic solvent-type adhesives are added to a powdery colorant and uncolored thermoplastic resin pellets in the step of mixing them in the above-mentioned conventional methods for preparing colored, shaping thermoplastic resin pellets, then the powdery colorant will adhere to the surfaces of the uncolored resin pellets. However, since a large amount of static electricity is generated in the mixer due to friction in the mixing step, the addition of organic solvents causes the danger of firing or explosion. Precisely, when thermoplastic resin pellets are mixed with colorants and additives in a mixer, a large amount of static electricity is generated instantaneously to give a discharging sound of sputtering or a flash so that the thus-charged mixer cannot be handled with hands. Therefore, in such a mixing step, addition of combustible organic solvents to the mixer causes the danger of firing or explosion so that organic solvents cannot be used for mixing thermoplastic resin pellets and powdery colorants. In this case, a means of adding antistatic agents to the mixer so as to prevent the generation of static electricity therein may be taken into consideration, but many antistatic agents are oily substances such as surfactants. Therefore, if such antistatic agents are added, the oily substances will adhere to the surfaces of the pellets to retard the adhesion of powdery colorants to the pellets. In place of this, another means of using incombustible chlorine-containing solvents may be employed with safety, by which, however, chlorine compounds will remain in the shaping thermoplastic resins prepared and these will cause other problems of corroding shaping molds or polluting the environment. Therefore, the latter is also unfavorable.